The short answer is that there is no way around a lost WEP key, if at the same time you have lost the admin password to the router at the same time.
However there is a workaround for your problem. (I will make a couple of assumptions here, please let me know if any of them are wrong and I will adjust my answer given the more complete information...
1. You are using a low end WIFI such as one made by D-link or Linksys
2. You are the legitimate administrator of this WIFI)
3. You have physical access to the WIFI unit
)
Quite simply, you need to do a factory reset in which case the admin password will return to the default.
At this point you can simply reset the WEP password as required.
After you do this, don't forget to document your passwords in a safe location.
BTW, the factory reset is different depending on brand, but usually consists of a reset button which needs to be pushed with a pin at the same time the unit is turned on. (Read your manual for your specific procedure... and if you have lost the manual, most manufacturer have a copy of the manual on line.)
